Pyrochlore iridates having metallic conductivity and their production method

ABSTRACT

The present invention was made to develop a new material possessing simultaneously the two properties of geometrically frustrated state having a magnetic controllability and good conductivity which leads to applications for electrical controllability and thermal storage. This object is achieved by the conductive material having the pyrochlore structure represented by the general formula R 2 Ir 2 O 7 , where R is a rare earth element or elements of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y. When, especially, R is an element or elements of La, Ce, Pr, Nd, Pm, Sm and Eu, the material possesses metallic electrical conductivity, so that it is useful for magnetically controllable functional electronic materials and thermal storage materials having large thermal capacity.

FIELD OF THE INVENTION

[0001] The present invention relates to an electronic functionalmaterial using the interaction between magnetic ions and electricconduction electrons in a crystal, and also to a thermal storagematerial having high thermal conductivity and large thermal capacity ina wide range of temperatures.

BACKGROUND ART

[0002] Conventional electronic functional materials using theinteraction between magnetic ions and electric conduction electrons in asolid include various magnetic memory elements and giantmagnetoresistive elements. The magnetic states of those functionalmaterials are the long range ordered states in which they areenergetically stable under given circumstances.

[0003] When the interaction between the geometrical arrangement of themagnetic elements on the lattice sites and the magnetic moments (orspins) of the magnetic elements satisfies a certain condition, the spinarrangement is not determined unique but many states having the sameenergy level degenerate even in the proximity of the absolute zerotemperature. This situation is termed as “geometrical frustration”.

[0004] In that case, though, the degenerated states separate again whenan external effect, such as an external magnetic field, is applied. Thismeans that the magnetic state of the crystal can be controlled byapplying an external effect such as an external magnetic field. Also, inthat case, it is known that other quantum effects such as an anomalousHall effect due to a local magnetic field may occur.

[0005] On the other hand, the crystal field effect is also important dueto the coupling of orbital angular momenta and spins of electrons. Owingto this effect, energy levels can be changed by a magnetic field, whichmeans that the magnetic states of a crystal can be controlled.

[0006] In oxides having the pyrochlore structure as shown in FIG. 1, thethree-dimensional network of corner-sharing tetrahedra (with an O atomat the corner) causes geometrical frustration when magnetic elements Rexist at the corners. When rare earth elements are used in this case,the crystal field effect is also important. That is, both thegeometrical frustration and the crystal field effect characterize thesystem.

[0007] In some oxides having the pyrochlore structure, the geometricalfrustration is also called as “spin ice”, because of the analogy in thespin arrangement of oxygen-magnetic ion system to the spatialarrangement of the oxygen hydrogen system in water ice (M. J. Harris etal., Phys. Rev. Lett. 79, 2554-2557 (1997)).

[0008] Known compounds of such kind include Ti pyrochlore oxides (e.g.,Ho₂Ti₂O₇: M. J. Harris et al., ibid; Dy₂Ti₂O₇: A. R. Ramirez, Nature399, 333-335 (1999)) and Sn pyrochlore oxides. But these oxides areinsulators, so that the practical use or application to electricalfunctional elements are quite limited.

[0009] Among Mo— (e.g., Y₂Mo₂O₇: M. J. P. Gingras et al., Phys. Rev.Lett. 78, 947-950 (1997)), Mn— and Ru-pyrochlore oxides, some areelectrically conductive, but they develop well known magneticallyordered states such as the spin-glass ordering or antiferromagneticordering due to disorders contained in those materials or due to thestructural phase transition, so that the large specific heat that shoulddevelop when the geometrical frustration exists does not appear.

[0010] As explained above, the state containing the geometricalfrustration can be called as a “magnetic state containingcontrollability” in the sense that the magnetic state can be controlledby applying an external magnetic field. In that case, though, itsapplication to industrial use is practically quite limited if it is notassociated with some device for controlling it or with some sensor fordetecting the magnetic state. Thus, for the purpose of industrialapplication, it is desired to develop a material which can show the spinice state, or similar state, and has a good electrical conductivity.

[0011] It is another advantage that such a material that develops nomagnetic transition to long range ordered magnetic state down to lowtemperatures has a large specific heat in a wide range of temperature.When such a material is intended to be used for a thermal storagematerial, it is strongly desired to have a large thermal conductivity toexchange heat with peripheral devices. From this point also it isdesired to have a good conductivity or to be metal, rather than aninsulator.

DISCLOSURE OF THE INVENTION

[0012] The inventors of the present invention have developed a newmaterial possessing the two useful qualities simultaneously: one thatshows the magnetically controllable state containing the geometricalfrustration, and the other that shows a good conductivity which leads toelectrical controllability and an application to thermal storage.

[0013] According to the present invention, the new material is thepyrochlore iridates (oxides) having metallic conductivity represented bythe general formula R₂Ir₂O₇, where R is an element or elements selectedfrom the group of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm,Yb, Lu and Y.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is an atom-ion configuration diagram of the pyrochlorestructure.

[0015]FIG. 2(a) is a tetrahedron composed of an oxygen atom and ions ofrare earth elements R, and FIG. 2(b) is an octahedron composed of aniridium and oxygen atoms, both drawn out from the R₂Ir₂O₇ pyrochloreoxide according to the present invention.

[0016]FIG. 3 is an atom-ion configuration diagram of the R₂Ir₂O₇pyrochlore oxide according to the present invention.

[0017]FIG. 4 is a graph showing the relationship between the temperatureand the specific heat of a pyrochlore oxide Pr₂Ir₂O₇ according to thepresent invention.

[0018]FIG. 5 is a graph showing the relationship between the temperatureand the electrical resistivity of various pyrochlore iridate oxidesaccording to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

[0019] The pyrochlore iridate oxides are represented by R₂Ir₂O₇, inwhich materials with Pb and Bi, both having no magnetic moment, as R areknown so far. Also known is that Eu₂Ir₂O₇ can be synthesized, but it hasbeen only known that Eu₂Ir₂O₇ is electrically conductive above the roomtemperature (R. J. Bouchard and J. L. Gillson, Mat. Res. Bull. 6,669-680 (1971)). The material of the present invention uses, as R, oneor a combination of elements from the group of rare earth elements,i.e., lanthanum (La, 57), cerium (Ce, 58), praseodymium (Pr, 59),neodymium (Nd, 60), promethium (Pm, 61), samarium (Sm, 62), europium(Eu, 63), gadolinium (Gd, 64), terbium (Tb, 65), dysprosium (Dy, 66),holmium (Ho, 67), erbium (Er, 68), thulium (Tm, 69), ytterbium (Yb, 70),lutetium (Lu, 71) and yttrium (Y. 39) (numbers in the parentheses areatomic numbers).

[0020] Generally, rare earth elements are stable in the compounds ofoxidation number three, and the ions have magnetic moment and showparamagnetism, except in the case of La³⁺, Ce⁴⁺, Lu³⁺, Yb²⁺ or Y³⁺,which are diamagnetic (“Dictionary of Physics and Chemistry 5th Ed.”,Iwanami, 1998). In the R₂Ir₂O₇ pyrochlore oxides of the presentinvention, the corners of the tetrahedra centering O are occupied by therare earth trivalent ions (FIG. 2(a), R₂O), and the octahedra composedof Ir and O are interpolated among them (FIG. 2(b), IrO₃×2=Ir₂O₆). Thusthe crystal structure is shown by FIG. 3.

[0021] The magnetic properties of those materials in the temperaturerange from the room temperature down to 10K can be explained well by thelocal magnetism due to the R³⁺ ions. From the measurements of magneticproperties and specific heat (FIG. 4) in further lower temperature rangeon Pr₂Ir₂O₇ which is a material of the present invention, Pr₂Ir₂O₇ showsa moderate peak in the specific heat which is apparently characteristicto the crystal field effect.

[0022] In a state having a magnetic frustration, a plurality of stateshaving the same energy provides degeneracy. When a disturbance, such asan external magnetic field of a certain direction, is applied to thestate, the energy levels of the states differ and the degeneracy islifted. In this case, the magnetic state of the material becomesdifferent from that before the external magnetic field or the like isapplied. By using some means for detecting it, the material can be usedas an electronic functional material such as a switch or memory. Sincethe intensity of the external magnetic field necessary to break thedegeneracy is generally very small, the magnetic device using thematerial of the present invention can have a very high sensitivity.

[0023] As shown in FIG. 5, when the ionic radius is relatively largesuch as in the case of Pr, Nd, Sm and Eu, the materials of the presentinvention exhibit metallic conductivity, where the electricalresistivity lowers as the temperature decreases. When, therefore, thoserare earth elements are used as R (generally, La, Ce, Pr, Nd, Pm, Sm andEu having smaller atomic numbers than Gd (64) except Y (39)), thematerials of the present invention develop metallic conductivity, andprovide possibility of application to the use as electronic functionalmaterial or thermal storage material.

[0024] When the ionic radius is relatively small (Gd, Tb, Ho, Yb and Yin FIG. 5. Generally, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu having theatomic number larger than Eu (63)), they show non-metallic conductivityin which the electrical resistivity increases as the temperaturedecreases. But in those cases, the actual value of the electricalresistivity is close to that of metals, so that they are classified tobe materials of good conductivity.

[0025] In the materials of the present invention, the electricalconduction is performed mainly by the tetravalent iridium ions Ir⁴⁺. Thetransition between metal and non-metal is attributable to the wideningof the energy gap by the electron correlation effect resulting from thedecrease in the width of the conduction band, which is caused by thedistorsion of crystal lattice due to the small R³⁺ ions.

[0026] The synthesizing method of the materials of the present inventionusing rare earth elements R is as follows. The starting materials ofoxides R_(n)O_(m) (where n and m are integers) and IrO₂ were mixed inthe stoichiometric ratio of R and Ir, and heated in air at temperaturesranging from 700C. to 1100C. (preferably from 800C. to 950C.) for fourdays. It is important to grind and mix it well every two days. SinceIrO₂ tends to sublimate, it is preferable to add some excess of itbefore or during the reaction for synthesizing purer materials.

[0027] The materials (oxides) of the present invention can be used as amagnetically controllable electronic functional materials having a goodelectrical and thermal conductivity in a powdered state or inpowdered-and-sintered state. It is also possible to grow their singlecrystals by a floating-zone technique, in which case the advantageousproperties are expected to develop intensely, and it is also expectedthat they are used as strong magnetic materials or electronic functionalmaterials.

[0028] According to the present invention, as described above, usingR₂Ir₂O₇ pyrochlore oxides composed of rare earth elements R andtransition metal element Ir, such a quantum state has been realized thatpossesses both highly controllable magnetic state including geometricalfrustration and metallic or non-metallic good electrical conductivity.The material of the present invention can be used in such applicationsas magnetic switching element which makes use of the strong magneticcontrollability susceptible even to faint external magnetic field, ormagnetic memory element. An application to electronic functionalmaterial is also possible using the quantum effect such as the anomalousHall effect which does not need an external magnetic field. Further itis expected to lead to the development of superconductive materialhaving an internal magnetic field and its applications.

[0029] The pyrochlore iridate oxides R₂Ir₂O₇ of the present inventionhave a large thermal capacity in a wide range of temperature at lowtemperatures. Some of them exhibit metallic properties so that they alsohave high thermal conductivity. Owing to these properties, the metallicmaterials according to the present invention and materials composedmainly of those materials are expected to be applied to thermal storagematerials necessary for cryocoolers or the like.

1. Magnetically controllable pyrochlore material having electricalconductivity represented by a general formula R₂Ir₂O₇, where R is anelement or elements selected from the group of rare earth elements La,Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y.
 2. Themagnetically controllable pyrochlore material having metallic electricalconductivity according to claim 1, wherein R is an element or elementsselected from the group of La, Ce, Pr, Nd, Pm, Sm and Eu.
 3. Themagnetically controllable pyrochlore material having metallic electricalconductivity according to claim 1 or 2, wherein R is Pr.
 4. A method ofproducing electrically conductive and magnetically controllablepyrochlore material comprising steps of: mixing starting materials ofoxides R_(n)O_(m) and IrO₂ in the stoichiometric ratio of R and Ir,wherein n and m are integers and R is an element or elements selectedfrom the group of rare earth elements La, Ce, Pr, Nd, Pm, Sm, Eu, Gd,Tb, Dy, Ho, Er, Tm, Yb, Lu and Y; and let them react in air attemperatures ranging from 700C. to 1100C. for about four days.
 5. Themethod of producing electrically conductive and magneticallycontrollable pyrochlore material according to claim 4, wherein thereacting temperature ranges from 800C. to 950C.
 6. The method ofproducing electrically conductive and magnetically controllablepyrochlore material according to claim 4 or 5, wherein reactant isstirred in the course of reaction.
 7. The method of producingelectrically conductive and magnetically controllable pyrochlorematerial according to one of claim 4-6, wherein excess IrO₂ is furtheradded in the course of reaction.
 8. A magnetic sensor used below thetemperature of 10K characterized in that it uses the magneticallycontrollable pyrochlore material according to claim 2 or
 3. 9. Amagnetic switching element used below the temperature of 10Kcharacterized in that it uses the magnetically controllable pyrochlorematerial according to claim 2 or
 3. 10. A magnetic memory element usedbelow the temperature of 10K characterized in that it uses themagnetically controllable pyrochlore material according to claim 2 or 3.11. A thermal storage element for cryocoolers characterized in that ituses the magnetically controllable pyrochlore material according toclaim 2 or 3.